Note that sexology is considered descriptive, not prescriptive: it attempts to document reality, not to prescribe what behavior is suitable, ethical, or moral. Sexology has often been the subject of controversy between supporters of sexology, those who believe that sexology pries into matters held sacrosanct, and those who philosophically object to its claims of objectivity and empiricism.

The scientific study of sexual behavior in human beings began in the 19th century. Shifts in Europe's national borders at that time brought into conflict laws that were sexually liberal and laws that criminalized behaviors such as homosexual activity.

In England, the founding father of sexology was the doctor and sexologist Havelock Ellis who challenged the sexual taboos of his era regarding masturbation and homosexuality and revolutionized the conception of sex in his time. His seminal work was the 1897 Sexual Inversion, which describes the sexual relations of homosexual males, including men with boys. Ellis wrote the first objective study of homosexuality, (the term was coined by Kertbeny) as he did not characterize it as a disease, immoral, or a crime. The work assumes that same-sex love transcended age taboos as well as gender taboos. Seven of his twenty-one case studies are of inter-generational relationships. He also developed other important psychological concepts, such as autoerotism and narcissism, both of which were later developed further by Sigmund Freud.

Ellis pioneered transgender phenomena alongside the German Magnus Hirschfeld. He established it as new category that was separate and distinct from homosexuality. Aware of Hirschfeld's studies of transvestism, but disagreeing with his terminology, in 1913 Ellis proposed the term sexo-aesthetic inversion to describe the phenomenon.

In 1908, the first scholarly journal of the field, Journal of Sexology (Zeitschrift für Sexualwissenschaft), began publication and was published monthly for one year. Those issues contained articles by Freud, Alfred Adler, and Wilhelm Stekel. In 1913, the first academic association was founded: the Society for Sexology.

Freud developed a theory of sexuality. These stages of development include: Oral, Anal, Phallic, Latency and Genital. These stages run from infancy to puberty and onwards. based on his studies of his clients, between the late 19th and early 20th centuries. Wilhelm Reich and Otto Gross, were disciples of Freud, but rejected by his theories because of their emphasis on the role of sexuality in the revolutionary struggle for the emancipation of mankind.

Pre-Nazi Germany, under the sexually liberal Napoleonic code, organized and resisted the anti-sexual, Victorian cultural influences. The momentum from those groups led them to coordinate sex research across traditional academic disciplines, bringing Germany to the leadership of sexology. Physician Magnus Hirschfeld was an outspoken advocate for sexual minorities, founding the Scientific Humanitarian Committee, the first advocacy for homosexual and transgender rights.

Hirschfeld also set up the first Institut für Sexualwissenschaft (Institute for Sexology) in Berlin in 1919. Its library housed over 20,000 volumes, 35,000 photographs, a large collection of art and other objects. People from around Europe visited the Institute to gain a clearer understanding of their sexuality and to betreated for their sexual concerns and dysfunctions.

Hirschfeld developed a system which identified numerous actual or hypothetical types of sexual intermediary between heterosexual male and female to represent the potential diversity of human sexuality, and is credited with identifying a group of people that today are referred to as transsexual or transgender as separate from the categories of homosexuality, he referred to these people as 'transvestiten' (transvestites). Germany's dominance in sexual behavior research ended with the Nazi regime. The Institute and its library were destroyed by the Nazis less than three months after they took power, May 8, 1933. The institute was shut down and Hirschfeld's books were burned.

Psychologist and sexologist John Money developed theories on sexual identity and gender identity in the 1950s. His work, notably on the David Reimer case has since been regarded as controversial, even while the case was key to the development of treatment protocols for intersex infants and children.

In 1966 and 1970, Masters and Johnson released their works Human Sexual Response and Human Sexual Inadequacy, respectively. Those volumes sold well, and they were founders of what became known as the Masters & Johnson Institute in 1978.

Vern Bullough was a historian of sexology during this era, as well as being a researcher in the field.

The emergence of HIV/AIDS in the 1980s caused a dramatic shift in sexological research efforts towards understanding and controlling the spread of the disease.